A multicarrier radio communication system supports radio communication between two nodes in the system, e.g. a user equipment, UE, and a radio base station, RBS, by providing a frequency spectrum of a plurality of subcarriers carrying communication between the two nodes.
There are different types of radio communication systems employing a multicarrier based solution for supporting communication between two nodes in the radio communication system. One example of such a type of a radio communication system is Long Term Evolution, LTE.
LTE uses Orthogonal Frequency Division Multiplex. OFDM, in the downlink and Discrete Fourier Transform, DFT, spread OFDM in the uplink. The basic LTE physical communication resources can thus be seen as a time-frequency grid, as illustrated in the example in FIG. 1a, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port).
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame including ten equally-sized subframes of 1 ms as illustrated in FIG. 1b. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive resource blocks (in time) represent a resource block pair and correspond to the time interval upon which transmission scheduling operates. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks, VRB, and physical resource blocks, PRB, has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localised and distributed. In the localised resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localised VRBs are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRB are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
FIG. 2 illustrates a mapping of physical control/data channels and signals onto resource elements within a downlink subframe. In this example, the Physical Downlink Control Channels, PDCCHs, occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start at the second OFDM symbol. Since the Cell-specific Reference Signal, CRS, is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE-specific RS (Demodulation Reference Signals, DMRS) where each UE has RS of its own placed in the data region of FIG. 2 as part of the PDSCH.
In the downlink, DL, of LTE, an unused Direct Current, DC, subcarrier in introduced. No information is mapped onto this subcarrier. For transmission bandwidths with an even number of resource blocks, the DC subcarrier is inserted between PRB N/2−1 and N/2, where N is the number of PRBs. For an odd number of PRBs, the DC subcarrier divides the PRB floor (N/2) into two halves.
Local oscillator leakage, mainly at the terminal side, may create large interference that after demodulations falls onto the DC subcarrier. Therefore, the DC subcarrier is introduced and no information is transmitted upon it.
Not all user equipments may be able to monitor the whole system bandwidth. In case a user equipment does not monitor the whole system bandwidth, but only a subset of it, such devices are limited to monitor the smaller bandwidth symmetrically around the DC subcarrier. This is due to leakage of the local oscillator of the UE, wherein the centre frequency of the local oscillator should coincide with the DC subcarrier. Such a UE typically relies on DMRS for data demodulation and Enhanced PDCCH (ePDCCH) which span only a fraction of the system bandwidth for control signalling.